The French documentary makers behind last night's harrowing "inside hell" film about the collapse of the twin towers admit they did not show the full horrors of the carnage around them.

Jules Naudet, who was with fire fighters in the lobby of tower one when tower two collapsed, said he saw two people engulfed in flames and dying before his eyes almost as soon as he entered the building.

They had caught fire after fuel from the American Airlines jet had gushed down the elevator shaft.

But Naudet said a kind of "auto-censorship" kicked in and he refused to film them dying out of respect.

Newspaper TV critics today applauded the documentary as a "magnificent" and powerful documentation of the cataclysmic events of September 11 2001. But several also criticised its descent in "schmaltz" towards the end of the film.

In a interview screened by BBC Interactive and available only to viewers with digital TV, the film makers, Jules and his brother Gedeon, explained how and why they shot the film.

"There was a very big controversy, in fact, before the first airing on CBS on March 10 because there were rumours going round the country that the footage that Jules and I had shot was absolutely horrible and that we had filmed carnage and body parts and people dying in front of the camera, but it is absolutely not what we did and refused to do that from the very beginning," said Gedeon.

He added: "So everyone was very much afraid of what they were expecting to see and when people had seen the documentary and most of the families called us to thank us."

The brothers' film of the first plane going into the twin towers was seen all over the world in the days after the attack - but the film makers said they didn't know for sure if they had captured the image because they too had felt momentarily paralysed by the shock of what had happened.

They were out with New York's fire fighters on a routine call following a suspected gas leak in downtown New York when the first plane hit.

"I remember filming fire fighters and then when we heard the plane and I had time to see the plane going between the buildings and could make out the American Airlines on it, it was that close and then I swung the camera round... I discovered much later on that I had filmed it," said Jules.

He was out with battalion chief Joseph Pfeifer getting some camera practice when the first plane hit. He explained how Chief Pfeifer - who lost his brother in the attacks - knew immediately it wasn't an accident as there was no smoke from the plane and it wasn't wobbling in the sky. "He made the deduction very quickly."

Although Jules and Gedeon were shooed away in the immediate aftermath by a New York police officer who screamed "This isn't fucking Disneyland", Chief Pfeifer instinctively knew the documentary was right. His decision to allow Jules follow him into tower one produced what one TV critic said today was "the closest we may ever come to experience it at first hand".

One of Jules' first traumas was the sound of bodies falling to the ground as they stood in the foyer filming fire fighters setting up a command centre and shepherding up to 25,000 people out of the towers.

"At first, like anyone I heard this huge sound. It was almost like a car that had been dropped from very high up and it could be a piece of the building falling down. and then the second one, the third one, and then I heard the firefighter behind me saying 'OK, we've got jumpers' then I realised this is actually a person we are hearing and dying as we hear the sound and it was constant because it was every 40 to 50 seconds," he said.

Jules also explained how the trauma sent him into auto-censorship mode

"I remember that day because we were confronted by these horrible images of death. As I entered this building I hear a scream on my right and I just turned and seeing that horrible image of two people who are still alive and moving but who are in flames and dying in front of my own eyes. The image was so horrible and so traumatising in a way and I didn't film it... it was so bad... there is some kind of respect that should be shown in death. That kind of auto-censorship, we did it for the rest of the day and the weeks to come... we always shied away from showing that.

"Nor did they film the bodies raining down from the floors above. It meant there was nothing too dreadful to discard when the painstaking process of editing 180 hours of footage down to two hours arrived.

"There wasn't anything that we really couldn't put on TV. We never filmed anybody dying or dead in front of us, we always refused."